1. Field of the Invention
This invention relates to a control system for an internal combustion engine, and more particularly to a control system for an in-cylinder direct injection internal combustion engine, which controls the operation of the engine such that the combustion mode of the engine is switched between a stratified combustion mode for performing stratified combustion of an air-fuel mixture, and a homogeneous combustion mode for performing homogeneous combustion of the air-fuel mixture, and that torque generated by the engine is controlled based on a desired torque.
2. Description of the Prior Art
Conventionally, a control system for an internal combustion engine, of the above-mentioned kind, was proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 10-103118. This internal combustion engine is installed on an automotive vehicle and includes two kinds of fuel injection valves, one for use in stratified combustion and the other for use in homogeneous combustion. Fuel injection valves for stratified combustion are each mounted in a cylinder head for a corresponding one of cylinders such that fuel is directly injected into a combustion chamber of the corresponding cylinder. Further, a fuel injection valve for homogeneous combustion is mounted in an intake pipe at a location upstream of a throttle valve such that fuel is injected into the intake pipe.
The control system controls the operation of the internal combustion engine such that the combustion mode of the engine is switched between a stratified combustion mode selected when the engine is in a low-load operating condition, and a homogeneous combustion mode selected when the engine is in a high-load operating condition, and that the amount of fuel controlled thereby is injected from the fuel injection valves for homogeneous combustion and stratified combustion to generate a required torque. More specifically, the control system determines as the required torque a total desired injection amount of fuel to be injected from the two kinds of fuel injection valves, based on operating conditions (engine rotational speed, accelerator pedal opening, etc.) of the engine, compares the total desired injection amount with a threshold, and switches the combustion mode between the stratified combustion mode and the homogeneous combustion mode according to results of the comparison.
In the homogeneous combustion mode, fuel injection is carried out by using the two kinds of fuel injection valves, and a desired fuel injection amount (hereinafter referred to as xe2x80x9cthe homogeneous desired injection amountxe2x80x9d) to be injected from the fuel injection valve for homogeneous combustion, and a desired fuel injection amount (hereinafter referred to as xe2x80x9cthe stratified desired injection amountxe2x80x9d) to be injected from one of the fuel injection valves for stratified combustion are determined in the following manner: A correction coefficient is determined based on operating conditions of the engine to multiply the total desired injection amount by the correction coefficient, and the resulting product of the multiplication is set to the homogeneous desired injection amount. Further, based on the homogeneous desired injection amount calculated this time and a smoothing value (averaging value) calculated last time, a present smoothing value is calculated, and a difference calculated by subtracting the present smoothing value from the total desired injection amount is set to the stratified desired injection amount. In the homogeneous combustion mode, the homogeneous desired injection amount and the stratified desired injection amount are set as described above.
On the other hand, in the stratified combustion mode, the homogeneous desired injection amount is set to xe2x80x9c0xe2x80x9d and the stratified desired injection amount is set to a difference calculated by subtracting the present smoothing value from the total desired injection amount, whereby fuel injection is carried out only by using the fuel injection valve for stratified combustion. When the operating condition of the engine becomes stable after the combustion mode is switched from the homogenous combustion mode to the stratified combustion mode, the smoothing value becomes almost equal to xe2x80x9c0xe2x80x9d, so that the total desired injection amount is set to the stratified desired injection amount, without any substantial subtraction.
In the control system constructed as above, when the total desired injection amount is increased with an increase in the required torque, the combustion mode of the engine is switched from the stratified combustion mode to the homogeneous combustion mode. As described above, the fuel injection valve for homogeneous combustion is located farther from the combustion chamber than the fuel injection valves for stratified combustion are, so that when the stratified combustion mode is switched to the homogeneous combustion mode, fuel from the fuel injection valve for homogeneous combustion is supplied to the combustion chambers with some delay, i.e. later than fuel from the fuel injection valves for stratified combustion. To eliminate this inconvenience, the homogeneous desired injection amount and the stratified desired injection amount are calculated, as described above, such that the stratified desired injection amount is increased by an amount of fuel corresponding to the above delay. This makes it possible to prevent the air-fuel ratio of an air-fuel mixture supplied to the engine and the output torque generated by combustion of the mixture from being largely changed upon switching of the combustion mode from the stratified combustion mode to the homogeneous combustion mode. Similarly, when the homogeneous combustion mode is switched to the stratified combustion mode in response to a decrease in the required torque, fuel is injected in the stratified desired injection amount which is the difference calculated by subtracting the smoothing value from the total desired injection amount, whereby a large change in the air-fuel ratio can be prevented.
According to the conventional control system, it is possible to prevent a large change in the air-fuel ratio when the combustion mode is switched between the stratified combustion mode and the homogeneous combustion mode, as described above. However, when the engine enters a relatively stable operating condition with the lapse of a transitional time period after the combustion mode is switched e.g. to the homogeneous combustion mode, there no longer occur significant changes in the total desired injection amount or the homogeneous desired injection amount, and the smoothing value also becomes almost constant, so that the smoothing does not produce almost any effect. Similarly, the stratified desired injection amount also ceases to undergo any significant change. For this reason, an actual sum-total desired injection amount which is the sum total of the homogeneous desired injection amount and the stratified desired injection amount both finally determined and injected respectively from the two kinds of fuel injection valves becomes almost identical to the total desired injection amount calculated based on operating conditions of the engine. As a result, the sum-total desired injection amount for actual injection comes to be sensitively changed in response to a slight change in the required torque, e.g. a subtle change in the engine rotational speed or the accelerator pedal opening, so that the air-fuel ratio is too sensitively changed, resulting in degraded drivability. Further, when a transitional time period has elapsed after the combustion mode is switched to the stratified combustion mode, the smoothing value becomes almost equal to xe2x80x9c0xe2x80x9d, so that the total desired injection amount is directly set to the stratified desired injection amount. Therefore, when a subtle change occurs in the required torque, the air-fuel ratio is sensitively changed in direct response to the subtle change, thereby causing the same problem as in the homogeneous combustion mode.
It is an object of the invention to provide a control system for an internal combustion engine which controls the operation of the engine such that the combustion mode of the engine is switched between a stratified combustion mode and a homogeneous combustion mode, and that there can be ensured stability of the air-fuel ratio of an air-fuel mixture supplied to the engine in response to subtle changes in required torque, thereby enhancing drivability.
To attain the above object, the present invention provides a control system for an in-cylinder direct injection internal combustion engine, which controls an operation of the engine such that a combustion mode of the engine is switched between a stratified combustion mode for performing stratified combustion of an air-fuel mixture, and a homogeneous combustion mode for performing homogeneous combustion of the air-fuel mixture, and that torque generated by the engine is controlled based on a desired torque.
The control system according to the invention is characterized by comprising:
operating condition detection means for detecting operating conditions of the engine;
required torque calculation means for calculating a required torque based on results of detection by the operating condition detection means;
combustion mode determination means for determining which of the stratified combustion mode and the homogeneous combustion mode should be set to the combustion mode, according to the required torque;
smoothed required torque calculation means for calculating a smoothed required torque by smoothing the calculated required torque, in dependence on the determined combustion mode; and
desired torque-setting means for setting the desired torque based on the calculated smoothed required torque.
According to this control system, a required torque is calculated based on operating conditions of the internal combustion engine, and the combustion mode of the engine is set to either the stratified combustion mode or the homogeneous combustion mode according to the required torque. Further, a smoothed required torque is calculated by smoothing the calculated required torque in dependence on the combustion mode determined, and a desired torque for use in carrying out the torque control is set based on the smoothed required torque. As described above, in each of the stratified combustion mode and the homogeneous combustion mode, the torque control is executed based on the smoothed required torque which is obtained by smoothing the required torque, so that differently from the conventional control system, even, when there occurs a subtle change in the required torque, it is possible to ensure stability of the air-fuel ratio of an air-fuel mixture supplied to the engine while preventing the air-fuel ratio from being adversely affected by the change in the required torque. This results in enhanced drivability. Further, for the same reason, it is possible to carry out the torque control while preventing noises in outputs from the operating condition detection means from adversely affecting the torque control.
Preferably, the required torque is smoothed to a greater degree when the combustion mode of the engine is set to the stratified combustion mode than when the combustion mode of the engine is set to the homogeneous combustion mode.
Generally, when the engine is in the stratified combustion mode, the air-fuel ratio of the mixture is controlled to a far leaner value than when the engine is in the homogeneous combustion mode, so that it is necessary to operate air intake devices, such as a throttle valve and an EGR (exhaust gas recirculation) valve, and further, the air-fuel ratio is more sensitively responsive to subtle changes in the required torque when the engine is in the stratified combustion mode, which necessitates more accurate control of the air-fuel ratio. According to the preferred embodiment, in the stratified combustion mode, the required torque is smoothed to a greater degree than in the homogeneous combustion mode, and hence it is possible to further enhance the stability of the air-fuel ratio while preventing the air-fuel ratio from being adversely affected by the subtle change in the required torque.
More preferably, the smoothing is carried out by calculating an average value of a predetermined number of values of the required torque calculated up to this time, the predetermined number being set to a larger value when the combustion mode of the engine is set to the stratified combustion mode than when the combustion mode of the engine is set to the homogeneous combustion mode.
Preferably, when a load on the engine is outside a predetermined range, the desired torque-setting means sets the desired torque based on the required torque instead of the smoothed required torque.
Preferably, the operating condition detection means includes at least one of an engine rotational speed sensor for detecting a rotational speed of the engine and an accelerator pedal sensor for detecting a degree of accelerator pedal opening.
Preferably, the engine includes a plurality of cylinders, and a plurality of fuel injection valves each permitting injection of fuel into a combustion chamber of a corresponding one of the cylinders, and the combustion mode of the engine is switched between the stratified combustion mode and the homogeneous combustion mode by changing timing of the injection of the fuel into the combustion chamber.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.